The present invention generally relates to vehicle brakes and more particularly relates to a method and a device for brake pressure adjustment of a hydraulic wheel brake.
In modern brake systems of vehicles, especially automotive vehicles, the brake pressures on the respective wheel brakes are controlled according to different parameters. A primary control circuit has control objectives such as optimization of deceleration, steering optimization, or stability optimization in an actual braking operation. To this end, the wheel brakes are activated according to different criteria or operating sequences, especially by determination of a defined nominal pressure in the brakes. This nominal pressure is adjusted in a secondary control. The secondary control, hence, receives the nominal pressure as an input quantity and produces actuating signals for valves, if necessary, brake force boosters or a hydraulic pump to adjust the nominal pressure. Among others, the actual pressure for the respective wheel brake is determined in order to produce therefrom, along with the nominal pressure, the deviation, with a view to taking further appropriate measures.
Therefore, brake pressure is not measured by sensor means in modern brake systems but is determined by way of a model. This is advantageous because it obviates the need for sensors and corresponding wiring structure.
The pressure model receives required input quantities and determines from these quantities, as well as according to system parameters, the actual pressure that prevails in the respective brake. More particularly, the pressure model can receive the control signals which influence the brake pressure on the brake under review, i.e., for example, signals for supply valves, discharge valves, for a hydraulic pump, or similar components. From these signals and from system parameters (for example, line cross-sections, viscosity of the hydraulic fluid, switching characteristics, etc.), the model can determine the actual pressure in parallel to the development of the pressure in the respective wheel brake so that the secondary control circuit may be closed by output of the actual pressure which is thus determined by way of the model.
One difficulty of existing systems is to take into consideration the influence of varying temperatures. The viscosity of the hydraulic fluid, usually hydraulic oil, declines at low temperatures. Principally, this influence can be recognized by further sensors. However, if it is desired to manage without additional sensors in this case, too, other strategies may be chosen. One strategy includes testing the value of the actual pressure determined by the pressure model with respect to a maximum system pressure. This is based on the following reflection: due to the increased viscosity of the (still cold) hydraulic oil, the existing pressure will lag behind the actual pressure determined by way of the model. This leads to the fact that the actual pressure determined by the model finally exceeds system limits which are not used in practical operations. It is detected in a like situation that the hydraulic fluid is still cold. This statement is taken into account in control strategies in an appropriate manner.
One shortcoming of this method is that much time passes by until the possibly critical condition is detected. This is shown by way of FIG. 3 where different pressure increase curves plotted against time are shown. Curve 320 is a curve which shows the pressure increase when the fluid is warm. Where the objective is to build up a brake pressure Psoll, it will build up along curve 320 with warm fluid and reaches the value Psoll at time t0. If, however, the hydraulic fluid is (still) cold, the increase takes its course along curve 310 due to the lower viscosity, and the value Psoll is reached only at time t1. Both curves 310 and 320 can rise qualitatively corresponding to an exponential function with a negative exponent because the further pressure increase depends on the difference in pressure between the pressure source and the system. In both cases, the end value would be the maximum possible system pressure as it can be produced, for example, by the hydraulic pump or the brake force booster. The brake pressure Pp corresponds to the maximum attainable brake pressure. The latter is generally not reached, alone because the brake system includes safety valves which open beforehand, for example, at the limit pressure Pgxe2x80x2. Therefore, the pressure in the brake system will not rise in excess of this value. The pressure Pgxe2x80x2 was used as a criterion for the poll, with the result of a slow identification.
The above disadvantage shows especially in modern traction slip control systems. In these systems, traction slip is frequently adjusted to desired values by way of brake intervention. Consequently, drive and brake operate in opposition to each other. This may become important, for example, in difficult driving maneuvers such as starting to drive on different coefficients of friction per vehicle side, especially when driving uphill. The wheels on one side can assume a high coefficient of friction and, hence, have a more or less normal grip, while the other wheels adopt a low coefficient of friction (snow or ice) so that they may a priori spin, with the result of positive slip being caused (wheel speed is quicker than vehicle speed). In the extreme case, no drive torque can be supplied to the vehicle because the entire propulsive power is conducted by way of the differential into the spinning wheel, and the wheel which has grip is not driven. Brake intervention at the spinning wheel causes a drive torque with which the vehicle can be furnished by way of the gripping wheel. This should be done as early as possible in order that the vehicle can be provided with a drive torque as early as possible. When time is lost then, this may have adverse effects, for example, that the vehicle rolls backwards, or that the engine and the brake operate in opposition to one another for an unnecessarily long time. On the other hand, a high amount of traction slip may be desirable to produce a supporting moment on the low coefficient-of-friction side.
An object of the present invention is to provide traction slip control which quickly identifies a wrong adaption of a pressure model without additional sensors.